1. Field of the Invention
The present invention relates to an injector driver, and specifically to an injector driver and a drive method therefor, which drive an injector of an internal combustion engine to which a battery directly supplies power.
2. Description of the Related Art
An electromagnetic fuel injector (hereinafter “injector”) is known as a conventional fuel injector in an internal combustion engine mounted aboard a vehicle such as an automobile. The injector includes a nozzle having a fuel injection port, a plunger, on the end of which is formed a valve (valve body), that is inserted into the nozzle and reciprocally moves freely therewithin, a return spring that imparts resilient force in the valve-closing direction to the plunger, and a coil that receives electrical power from a battery and provides electromagnetic force in the valve-opening direction to the plunger. By electrically powering the coil the plunger is pulled inward to move the valve away from the valve seat of the fuel injection port, thereby injecting fuel from the fuel injection port. When the electrical power to the coil is stopped, however, the magnetic attraction by the coil attenuates, and the resilience force of the return spring closes the valve.
In recent years, injectors (fuel injectors) have come to be disposed in the cylinders of gasoline engines to improve the combustion efficiency, and attempts have been made to inject fuel directly into a cylinder. By directly injecting fuel into a cylinder, because gasoline fuel supplied by an injector is entirely supplied to the cylinder, it becomes possible to perform combustion with a value that is closer to the theoretical value, and it is possible to reduce a fuel consumption and to achieve a reduction in NOx and hydrocarbons and the like contained in the exhaust gas.
In the case of direction injection, however, the space into which the gasoline fuel is injected is the space formed by the cylinder block, the piston, and the cylinder head and, if injection during the compression stroke is considered, combustion must be done at a pressure that a much higher than the case of injection into the intake manifold. Also, there is not enough space and time for the fuel to diffuse after it is injected. Under this type of condition, therefore, in order to achieve combustion conditions equivalent to those in past art, it is necessary to make the fuel pressure of the gasoline fuel supplied to the injector high, and to sufficiently diffuse the fuel within the cylinder from the instant of injection. This makes it necessary to perform high-speed drive of the injector to oppose the high fuel pressure, and also to perform accurate control of the fuel injection time. The driving circuit to achieve this must apply a high voltage in a short period of time to the injector (more precisely, to the injector solenoid) and must perform high-speed opening and closing of the needle valve of the injector.
The Japanese Patent Application Publication No. JP-A-11-351039, for example, discloses art wherein, in an injector drive circuit direct cylinder-injection engine, because a high fuel pressure is applied to the injector, a high magnetic attraction is required by the coil of the injector, rather than using battery voltage (+B) drive, the battery voltage (+B) is generally increased to approximately 50 to 200 V by a voltage-boosting unit and applied to the injector to operate the injector, after which a switch is made to a holding current.
In the art disclosed in Japanese Patent Application Publication No. JP-A-11-351039, however, although there are the advantages of the valve opening response time (T0) of the injector being short and the fact that there is no influence from a variation in the battery voltage (+B), it is necessary to use a voltage boosting unit to increase the battery voltage, and necessary to take noise countermeasures because of the use of a high voltage, thereby leading to the problem of an increase cost of the apparatus.
To solve the above-described problem, Japanese Patent Application Publication No. JP-A-2001-41085 discloses art in which, in driving the injector by the battery voltage (+B), a threshold value at which a switch is made to constant current control is changed depending upon the battery voltage (+B), and the threshold value is set smaller the lower is the battery voltage (+B), thereby preventing excessive current when the battery voltage is low.
However, in the case such as in Japanese Patent Application Publication No. JP-A-2001-41085, in which the injector drive current is controlled by the battery voltage (+B) without using a voltage-boosting unit, the voltage applied to the injector is reduced, making it difficult to suppress the variation (or make constant) of the injector valve opening response time T0 because of battery voltage variation and variations characteristic to each cylinder.
The variation of the injector valve opening response time T0 will be described with reference made to FIG. 10 to FIG. 12. FIG. 10 of the accompanying drawings describes the relationship between the voltage applied to the injector and the injector valve opening response time T0. In this drawing, the horizontal axis represents the voltage applied to the injector INJ, and the vertical axis represents the injector INJ valve opening response time T0. The symbol A denotes drive by the battery voltage (+B), and the symbol B denotes drive by the use of a voltage boosting unit.
In the case of driving using a voltage-boosting unit, as described above, even if the applied voltage varies, the span of change ΔT0 of the valve-opening response time T0 of the injector INJ is small, and there is no particular problem. In contrast, when driving using the battery voltage (+B) only, when the voltage applied to the injector INJ varies, the span of change ΔT0 of the valve-opening response time T0 of the injector INJ becomes large. The voltage applied to the injector INJ varies in accordance with variation of the battery voltage (+B) and variation in the coil resistance (including the wiring harness resistance) caused by ambient temperature variations and the elapse of time.
FIG. 11 describes the valve-opening response time T0 of the injector for the case in which the injector is controlled by a constant voltage, and FIG. 12 describes the valve-opening response time T0 of the injector for the case in which the injector is controlled by a constant current. As shown in FIG. 11 and FIG. 12, the current increasing tendency of the current flowing in the injector INJ differs between the case in which a high voltage is applied and the case in which a low voltage is applied. The valve-opening response time T0 of the injector INJ changes greatly depending upon the current increasing tendency of the current flowing in the injector INJ. In this manner, in the conventional constant-voltage control method and constant-current control method in which a voltage-boosting circuit is not used, there is the problem of not being able to suppress variation in the valve-opening response time T0.